Belgian Pat. No. 869,185, issued Jan. 22, 1979 (corresponding to U.S.S.N. 817,647, filed July 21, 1977, now abandoned), describes a novel catalytic material comprising carbon fibers containing small nodules of iron or another Group VIII metal. This material is formed by contacting the Group VIII metal with a mixture of carbon monoxide and hydrogen at elevated temperature to cause deposition of carbon through the disproportionation of carbon monoxide. When the material so formed is contacted with hydrogen at elevated temperature, the carbon in the material reacts with the hydrogen to form methane. Repeated cycling between carbon deposition and methanation causes the catalytic material to exhibit high activity, i.e. high carbon deposition rates and high methanation rates.
In addition to very high catalytic activity, this material exhibits other unusual properties. For example, the material undergoes drastic changes in volume between the carbon deposition the methanation steps. Thus, it has been found that the volume of the material at the end of a carbon deposition step can be as much as 20 times as great as the volume at the end of a methanation step. Also, it has been found that repeated carbon deposition/methanation cycling causes gradual pulverization of the Group VIII metal on which the carbon fibers are grown. Also, the carbon fibers, especially when grown from such pulverized metal can exhibit extremely great expansive forces. For example, in one instance a stainless steel laboratory reactor was actually split open by the carbon fibers grown therein when the volume of the carbon fibers exceeded the internal volume of the reactor.
Because of the high catalytic activity of this material, it has been proposed to use this material for the production of high BTU gas from coal. This would be accomplished by burning the coal to produce producer gas and reacting the carbon monoxide in the producer gas in accordance with the above technique to produce methane. Unfortunately, the unusual properties of this material make it difficult or impossible to handle in conventional gas/solid contacting apparatus such as fixed or fluid-bed reactors wherein a bed of the solid is continuously contacted with a gas flowing therethrough.
For example, in fixed-bed operation, destruction of the reactor by growing too much carbon fibers is always a possibility. Also, the powdery residue produced by repeated carbon deposition/methanation cycles tends to collect together and on subsequent carbon deposition steps the carbon/metal mass produced agglomerates and plugs the reactor inlets and outlets.
In fluid-bed operation, it is necessary that the solid being contacted has a relatively specific particle size distribution and density. The vast changes in volume as well as density of the material as well as the fragile nature of the carbon fibers makes the material totally unsuitable for fluid-bed operation.
Accordingly, it is an object of the present invention to provide a new catalytic material which is also capable of catalyzing carbon deposition and methanation reactions with high catalytic activity but which also has a combination of physical properties making it amenable for use in a conventional fixed-bed reactor.